Safety regulations for protecting vehicle passengers as well as fuel efficiency regulations for protecting the environment have recently been tightened, and thus there is increasing interest in techniques for improving the stiffness of automobile components and reducing the weight of automobiles. For example, along with attempts to reduce the weight of parts such as pillar reinforcing members or cross members forming passenger safety cage zones in automobiles as well as side members or front/rear bumpers forming crash zones in automobiles, the use of high-strength parts has been increased to guarantee stiffness and crashworthiness.
In automotive steel sheets, the increase of strength may inevitably result in the increase of yield strength, decrease in elongation, and significantly decreased formability. Thus, as a forming method for solving problems related to the formability of high-strength steel and providing high-strength automotive parts having a tensile strength grade of 1470 MPa or greater, a hot press forming method or a hot forming method has been developed and widely used.
Hot press forming guarantees various degrees of strength. For example, in the early 2000s, hot press formed products having a tensile strength grade of 1500 MPa could be manufactured using 22MnB5 steel, as stated in DIN. In general, before hot press forming process, a steel sheet blank having a tensile strength of 500 MPa to 800 MPa is heated to a temperature within an austenite temperature range of an Ac3 transformation temperature or higher and is transferred to the press equipped with a cooling device to form the blank and quench the press formed blank (product) in the dies. Therefore, a press formed product ultimately contains martensite or a mixture of martensite and bainite, and thus the press formed product may have ultra-high strength, on the level of 1500 MPa or greater. In addition, since a press formed product is rapidly cooled within dies, the press formed product may have precise dimensions.
The basic concept of the hot press forming method and the use of boron bearing steel in the hot press forming method were first proposed in Patent Document 1 (UK Patent No. 1490535) and have subsequently been widely used. In addition, an aluminum or aluminum alloy coated steel sheet has been proposed in Patent Document 2 (U.S. Pat. No. 6,296,805) to suppress the formation of surface oxide layer during heating in the hot press forming process. In addition, Zn-coated galvanized or galvannealed steel sheets have been proposed for applications which required sacrificial protection such as wet area of automotive body.
In addition, so as to improve the fuel efficiency of automobiles, automobile manufacturers have been increasingly interested in the higher tensile strength grade of steel sheets for hot press forming. In this regard, a steel sheet for manufacturing a hot press formed product having a tensile strength grade of 1800 MPa has been proposed. Compared to steel sheets for manufacturing hot press formed products having a tensile strength grade of 1500 MPa, the proposed steel sheet has a relatively high carbon content, and niobium (Nb) effective in refinement of initial austenite grains is added to the proposed steel sheet to improve the toughness of hot press formed products.
However, the above-described methods of the related art for improving the strength of hot press formed products result in the formation of cracks, an increase in susceptibility to crack propagation, and accordingly, poor bendability.